As used herein, the term "virus" refers to a simple infectious organism that is an obligatory intracellular parasite. The term "virus particle" is a bloc of genetic material (either DNA or RNA) capable of autonomous replication, surrounded by a protein coat and sometimes by an additional membranous envelope which protects the virus particle from the environment and serves as a vehicle for its transmission from one host cell to another. The term "host" is an animal or human which is infected by a specific type of virus. The term "host cell" is a cell that is infected by a virus particle.
Viral infections are difficult to treat in animals and man due to the unique manner in which viruses utililize a host's own metabolic machinery to replicate new virus particles. Unlike cells, viruses do not grow in size and then divide because they contain within their coats few or none of the biosynthetic enzymes and other machinery required for their replication. Rather, viruses replicate in host cells by synthesis of their separate components and then assembly of the individual components. Thus, the viral nucleic acid, after shedding its coat, comes in contact with the appropriate cell machinery. The viral nucleic acid then specifies the synthesis of proteins required for viral reproduction. The viral nucleic acid is itself replicated, through the use of both viral and host cell enzymes; the components of the viral coat are synthesized; and these two components are then assembled to form a complete virus particle, also called a virion.
Because viruses use a host cell's enzymatic machinery to replicate, viral infections cannot be treated as infections caused by cellular microorganisms. Chemicals that will block the replication of viruses will also block the enzymes that are required for a host cell to live, thereby damaging or killing the host cell. For this reason, virus infections cannot be treated using conventional antimicrobial drugs.
Viruses are subdivided into three main classes; animal viruses, bacterial viruses, and plant viruses. Within each class, each virus is able to infect only cells from certain species. The host range is determined by the specificity of attachment to the cells. This depends on properties of both the virus' coat and specific receptors on the host cell's surface.
Most viruses that naturally infect a particular species of animal will usually be controlled by the animal's immune system. After a virus infects an animal, the animal's immune system will eventually recognize the presence of the virus and be stimulated to remove or neutralize the infecting viral particles. One of the problems with the natural system is that it takes the immune system time to recognize the virus and to become sufficiently stimulated so as to effectively remove the virus from the system. During the time that the immune system is being stimulated, the infecting virus can cause great damage. For example, viral infections may cause damage to vital tissues, such as brain, nerves, liver, etc. The crippling sequelae of poliomyelitis virus infections of humans, the muscle tics which follow distemper infections of dogs, and fibrosis and cirrhosis of the liver subsequent to viral hepatitis in humans, are but examples of permanent injury which may be caused by viral infections before the immune system has had sufficient time to develop an adequate response. Death frequently occurs in fulminating infections before the immune system can respond to the infectious agent.
Viral infections commonly compromise a host's defenses to other organisms, especially opportunistic bacteria, with a result that many of the signs and symptoms characteristic of many diseases are actually caused by these secondary invaders. A classic example of this type of secondary infection is bacterial infection which follows viral infection of respiratory tissues. Furthermore, other signs and symptoms of many viral infections are caused by inflammatory and toxic factors released by infected cells and cells which have died subsequent to viral infection. Multiple replicatory cycles of viral propagations usually are necessary to produce sufficient amounts of these factors to significantly affect a host.
Conventional therapy for preventing future viral infections includes immunization against a particular virus. Immunization against a virus requires the introduction of either live, attenuated (inactivated) virus or dead virus particles into the body. The body will recognize the virus and mount a specific immune response against the virus. After immunization against the virus, the immune system of the body will readily attack and neutralize an invading virus because the body has been previously primed by the inactivated or killed virus and will "remember" that particular virus. However, the protection is only effective against the virus to which the body had been immunized. Effective protection by immunization usually requires at least two to four weeks. Often, a second or even third "booster" shot will be required to maintain maximum immunization. Although immunization provides long term protection against future infection by a virus, immunization is ineffective against current, ongoing infections. Immunization is also only effective against the virus that was used to immunize the animal.
Thus, what is needed is a therapeutic agent that is capable of rapidly stimulating the immune system, in vivo, in such a way as to cause the immune system of the host animal to neutralize the virus, or interrupt its replication by recognizing and killing infected cells so that it can no longer infect other host cells.